The goal of the proposed research is to determine the influence of transcranial direct current stimulation (tDCS) on long-term motor learning, transfer of motor learning, and cortical function in Parkinson?s disease (PD). PD is the second most common neurodegenerative disorder and affects approximately 1 million people in the United States with total annual costs approaching 11 billion dollars. Current medical and treatment approaches for PD are either only mildly effective, expensive, or associated with a variety of side effects. Therefore, the development of practical, safe, affordable, and effective therapeutic adjuncts to current treatment approaches would have significant benefits. Recently, tDCS has emerged as a promising tool to enhance motor performance and cortical excitability in PD and may represent such an intervention with a realistic potential to be translated into clinical practice. This ability of tDCS to safely and reliably alter cortical excitability is important because increased cortical excitability following practice of a task has been interpreted as an indicator of use-dependent plasticity in the motor cortex and associated with improvements in motor function. Accordingly, tDCS applied to the motor cortex improves performance in a variety of motor tasks in healthy subjects, older adults, stroke, and in PD. However, these were acute studies that measured performance when cortical excitability was transiently increased during and after tDCS. Despite these promising preliminary findings, lack of available research on several crucial issues currently prevents the determination of the viability of tDCS as an adjunct intervention in PD. For example, the magnitude to which tDCS may be able to improve long-term motor learning beyond what can be achieved by practice alone in PD is unknown, although recent observations from long-term tDCS studies in young adults suggest this is possible. Furthermore, it is unclear if short-term improvements in motor function induced by tDCS and measured in the OFF state in PD can be attained over the long-term in the ON state, which is a necessary prerequisite for real world application. In addition, it is uncertain if the effects of tDCS generalize to non- practiced tasks or tasks not done in association with tDCS. Similarly, it is unknown if long-term application of tDCS to the left hemisphere leads to an improvement in performance of the non-stimulated, ipsilateral hand or if these effects are limited to the stimulated, contralateral hand only. Finally, the physiological mechanisms underlying any of these issues have not been identified because no long-term motor learning tDCS studies in PD to date have concurrently quantified behavioral, clinical, and physiological measures. The proposed project simultaneously addresses all of these critical obstacles through a 2 week training study that will involve tDCS applied during practice motor tasks with behavioral, clinical, and physiological evaluations at baseline as well as 1, 14 and 28 days following training. The findings of the proposed studies should have significant clinical significance and applications to comprehensive intervention therapy development in the treatment of PD. .